Radioisotopic generator

ABSTRACT

A radioisotopic or radionuclide generator comprises a radioisotope as a heat source and sealed within a thin-walled metal cylinder. Thermoelectric energy converters are arranged in heat transfer relation with the metal cylinder, and a thermal insulation surrounds the metal cylinder and the energy converters. The heat source, the energy converters and the thermal insulation are sealed within a thick-walled metal safety capsule whose exterior surface may be provided with a coat of depleted uranium to absorb gamma rays. The thick-walled sealing capsule has a central cylindrical portion and two hemispheric end portions welded or otherwise integral with the central cylindrical portion. The hemispheric ends of the capsule are designed as heat bridges for the energy converters. The thermal insulation is in the form of a cylinder whose mid-portion comprises a plurality of thin radially spaced metal foils of high thermal reflecting power, the spaces between the metal foils being highly evacuated. The end portions of the heat insulating cylinder comprise fibrous heat-insulating material, such as quartz cotton wool.

llnited States Patent [1 1 Merges [4 Aug. 28, 1973 RADIOISOTOPICGENERATOR [75] Inventor: Veit Merges, Munich, Germany [73] Assignee:Messerschmltt-Bolkow-Blohm Gesellschaft Mlt Beschrankter Haftung,Munich, Germany [22] Filed: Nov. 7, 1969 [21] Appl. N0.: 874,820

[30] Foreign Application Priority Data Nov. 23, 1968 Germany P 18 10528.7

[52] US. Cl. 136/202 [51] Int. Cl. G2lh 1/10 [58] Field of Search136/202 [56] References Cited UNITED STATES PATENTS 3,075,030 l/1963 Elmet a1. 136/202 X 3,272,658 9/1966 Rush 136/202 X 3,347,711 10/1967Banks, Jr. et a1. 136/202 3,357,866 12/1967 Belofsky 136/202 3,401,0649/1968 Perlow et a1 t 136/202 3,472,702 10/1969 Yeats et al 136/202OTHER PUBLICATIONS TID-22350, Nov. 1965, pp. 18, 19, 62-65, 67-69, 7l,72, 78, 81, 86, 87.

Primary Examiner-Harvey E. Behrend Attorney-McGlew & Toren [57] ABSTRACTA radioisotopic or radionuclide generator comprises a radioisotope as aheat source and sealed within a thinwalled metal cylinder.Thermoelectric energy converters are arranged in heat transfer relationwith the metal cylinder, and a thermal insulation surrounds the metalcylinder and the energy converters. The heat source, the energyconverters and the thermal insulation are sealed within a thick-walledmetal safety capsule whose exterior surface may be provided with a coatof depleted uranium to absorb gamma rays. The thickwalled sealingcapsule has a central cylindrical portion and two hemispheric endportions welded or otherwise integral with the central cylindricalportion. The hemispheric ends of the capsule are designed as heatbridges for the energy converters. The thermal insulation is in the formof a cylinder whose mid-portion comprises a plurality of thin radiallyspaced metal foils of high thermal reflecting power, the spaces betweenthe metal foils being highly evacuated. The end portions of the heatinsulating cylinder comprise fibrous heatinsulating material, such asquartz cotton wool.

5 Claims, 2 Drawing Figures Patented Aug. 28, 1973 INVENTOR Veit Mergesy ,jz fdif M ATTORNEYS RAPIOISOTOPIC GENERATOR BACKGROUND OF THEINVENTION Known radioisotopic generators, particularly for theproduction of electric power utilizing a radioisotope as a heat source,include an enclosure for the radioisotope, thermoelectric energyconverters, heat insulation, and heat collecting plates for eliminatingthe heat traversing the energy converters, such as shown, for example,in German Auslegeschrift 1216955. In these known radioisotopicgenerators, the sealing enclosure of the radioisotope is designed as athick-walled safety tank, and the energy converters are positioned onthe exterior of this safety tank.

Since radioisotopic generators work at operating temperatures of up to2000C, the safety tank must consist of highly refractory material, tomake certain that no radioactive material is released under anycircumstances.

In addition, the safety tank must be corrosionresistant at these hightemperatures over a long period, such as decades, or for as long as theenclosed radioisotope emits dangerous radiation.

Since the safety tank also must inclose the radioisotope safely againstaccidents, for example, falls, it must be designed, at the same time, asa pressure tank.

These three requirements regarding the stability of the capsule orradioisotope enclosure to great heat, corrosion and pressure, limit theselection of the material for the safety tank and increase,additionally, the costs of such radioisotopic generators.

Another disadvantage of known radioisotopic generators is that theenclosure for the radioisotope, which must have very thick walls becauseit must also function as a safety tank, increases the surface of theheat source and thus the heat losses.

With reference to the foregoing, a definition of certain terms would behelpful. Thus, according to the Code of Safety Practice on the Design,Installation and Use of Radioisotope Power Generators," published by theInternational Atomic Energy Agency in June l96 8,'the term capsuledenotes a device which effects the complete and tight sealing of theradioisotope and which can withstand all transportation loads,operational loads and accident loads without releasing its radioactivecontents. Specifically, a capsule is defined as a fuel envelope designedto prevent any dispersion of radioactive material.

In addition, a radioisotopic generator has a so-called generatorhousing" which holds together mechanically the above-mentioned so-calledhot capsule, the thermoelectric energy converter means, the heatinsulation, and the like. All known isotopic generators, such as, forexample, SNAP-3, SNAP-7, URIPS, SNAP-21, SNAP-23, MARGUERIT, and RIPPLE,are built in accordance with the concept outlined above.

SUMMARY OF THE INVENTION This invention relates to radioisotopicgenerators, particularly for the production of electric power, and,

more particularly, to an improved and simplified radio-- isotopicgenerator in which the capsule necessary for safely enclosing theradioisotope contains, in addition to the radioisotope. the energyconverters and the thermal insulation. I

The objective of the invention, based on known radioisotopic generators,is to provide a radioisotopic generator which also meets the usualsafety require ments regarding the enclosure of the radioisotope, butwhich is not subject to the restrictions, with respect to the selectionof the material for the safety tank, and where the heat source is small.In accordance with the invention, this problem is solved in that thecapsule, required for the safe enclosure of a radioisotope, con tains,in addition to the radioisotope, the energy converters and the thermalinsulation.

In accordance with the preferred embodiment of the invention, the metalcylinder surrounding the radioisotope is thin-walled, and it has two endfaces which are operatively associated, or in heat transfer relation,with the hot sides of the thermoelectric energy converters which aresurrounded, together with the metal cylinder, by a concentricthick-walled heat insulating cylinder. This heat insulating cylinder isopen at both ends, and the end zones are associated with heat bridgeswhich are parts of a thick-walled safety capsule. The safety capsule issealed all around and surrounds the entire arrangement, and hasextending therethrough only the insulated electric conductors. Theseheat bridges hold the energy converters, positioned adjacent oppositeends of thr radioisotope, through the medium of heat collecting plateswhich are in good thermal contact with the cold sides of the energyconverters.

Such a design has many advantages. Thus, the entire surface of thesafety capsule can be cooled directly from all directions, so that it isheated only slightly above the ambient temperature, independent of thestate of the energy converters. For this reason, it is possible to usematerials, for the construction of the thickwall safety tank, which neednot meet as high requirements as hitherto considered necessary withrespect to heat resistance and corrosion resistance at hightemperatures.

Another advantage is that the safety capsule at the same time protects,in this type of radioisotopic genera tor, the thermoelectric energyconverters against external influences such as pressure or shock. It isthus no longer necessary, when using the radioisotopic generators atgreat depths in the sea, to surround the generators with pressureresistant shells.

Since the cylinder sealingly enclosing the radioisotope does not have tomeet the above-mentioned safety requirements, it can have very thinwalls. The surface of the heat source is thus considerably smaller thanin known radioisotopic generators utilizing the same quantity ofradioisotope as a fuel.

In accordance with a further feature of the invention, the capsuleenclosing the radioisotope, the energy converters and the thermalinsulation is highly evacuated, so that the danger of corrosion for allmetal parts within the safety capsule is thus even further reduced,including reduction of the danger of corrosion of the inner wall of thesafety capsule.

In a preferred embodiment of the invention, the safety capsule consistsof a tubular or cylindrical center piece with two welded-onhemispherical heads or caps. The tubular center piece and the caps areso designed that they enclose the heat insulating cylinder.

As a heat insulating material, there are provided several layers of thinmetal foils, of high thermal reflecting power, whose interspaces arehighly evacuated. However, it is also possible to use fibrousheat-insulating material, for example, glass wool.

As a further feature of the invention, the heat bridges are designed ashollow cylinders so as to define cavities within the safety capsule.Within these cavities there can be arranged, if necessary, consumers ofthe electric energy supplied by the radioisotopic generator, or otherauxiliary devices.

Within the scope of the invention, either the capsule per se or togetherwith an additional coat on its inner or outer surface, for example, maybe provided with an additional coat of depleted uranium for theabsorption of gamma rays, this coating being designed as a screenagainst the radiation of the radioisotope. Furthermore, in place ofthermoelectric energy converters, it is also possible to use thermoionicconverters.

An object of the invention is to provide an improved radioisotopicgenerator particularly for the production of electric power using aradioisotope as a heat source.

Another object of the invention is to provide such a radioisotopicgenerator which meets the usual safety requirements with respect toenclosure of the radioisotope but which is not subjected to the usualrestrictions with respect to the selection of the material for thesafety tank.

A further object of the invention is to provide such a radioisotopicgenerator in which the capsule, required for sealing of a radioisotope,contains, in addition to the radioisotope, the energy converters and thethermal insulation.

Another object of the invention is to provide such a radioisotopicgenerator in which the radioisotope is sealed within a thin-walled metalcylinder having end faces in heat transfer relation with the hot sidesof ther moelectric energy converters.

A further object of the invention is to provide such a radioisotopicgenerator in which the enclosure of the radioisotope and the energyconverters are surrounded by a concentric thick-walled heat insulatingcylinder open at both ends.

Another object of the invention is to provide such a radioisotopicgenerator including heat bridges associ ated with the ends of the heatinsulating cylinder and forming parts of a thick-walled safety capsulesealed on all sides and surrounding the entire arrangement.

For an understanding of the principles of the invention, reference ismade to the following description of a typical embodiment thereof asillustrated in the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING In the drawing:

FIG. 1 is a longitudinal or axial sectional view through a radioisotopicgenerator embodying the invention; without the generator housing andFIG. 2 is a section along the line II-II of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT In the radioisotopic generatorillustrated in FIGS. 1 and 2, four disks 1, of a radioisotope, arestacked to form a cylinder which is surrounded by a thin-walled metalcylinder 2 having end faces 3 and 4. A thick heatinsulating cylinder 5,open at both ends, surrounds metal cylinder 2 and is arranged coaxiallywith the latter. The edge end zones 7 and 8 of heat-insulating cylinderextend beyond end faces 3 and 4 of metal cylinder 2.

The central portion of heat-insulating cylinder 5 consists of numerousthin metal foils of high thermal reflecting power, and the space betweenadjacent metal foils is highly evacuated. In its two edge zones 7 and 8,cylinder 5 consists of fibrous heat-insulating material such as, forexample, quartz wool.

The end zones 7 and 8 of heat-insulating cylinder 5 adjacent therespective end faces 3 and 4 of metal cylinder 2 define cavities inwhich there are positioned respective thermoelectric energy converters 9and 10 connected in cascade. The hot sides of these converters face therespective end faces 3 and 4 of metal cylinder 2. However, between theend faces 3 and 4 and the hot sides of the respective thermoelectricenergy converters 9 and 10 there are left expansion gaps 11 and 12 whichcan be filled with a material of good thermal conductivity but which iselastic by virtue of its geometric form. For example, this could be afibrous blend or a wool of highly refractory material, or an electricalinsulating material of good. thermal conductivity.

Thermoelectric energy converters 9 and 10 are held in position bycircular heat collecting plates 13 and 14, respectively, which are ingood thermal and electrically insulated contact with the cold sides ofthe energy converters 9 and 10. To this end, the heat-collecting plates13 and 14, whose diameter is equal to the internal diameter ofheat-insulating cylinder 5 which secures them against lateraldisplacement, cooperate with metal heat bridges l5 and 16 which formfixed parts of a thick-walled safety capsule 17.

Safety capsule l7 encloses the entire arrangement, and has extendingtherethrough, in sealed relation, only the insulated electric conductorsl8 and 19 for the thermoelectric converts. This safety capsule consistsof a tubular or cylindrical center piece 20 and two welded-onhemispherical caps or heads 21 and 22. The inside diameter of centerpiece 17 is equal to the outside diameter of heat-insulating cylinder 5,so that the latter is firmly positioned by safety capsule 17. Cavities23 and 24, defined by heat bridges l5 and 16, are in the shape of hollowcylinders and serve furthermore to receive consumers of electric energysupplied by the radioisotopic generator, and which are not shown, or toreceive other auxiliary devices.

Safety capsule 17 is covered by an additional coat 25 of depleteduranium for the absorption of gamma rays. Naturally, the surface ofcapsule 17 can be increased by convection plates and, instead of therepresented capsule form, it is also possible to use a different capsuleform, for example, a sphere.

While a specific embodiment of the invention has been shown anddescribed in detail to illustrate the application of the principles ofthe invention, it will be understood that the invention may be embodiedotherwise without departing from such principles.

What is claimed is:

l. A radioisotope capsule, forming a sealed safety enclosure for aradioisotope and capable of withstanding all transportation, operationaland accident shocks without release of its radioactive content, saidcapsule comprising, in combination, a thin-walled metal cylinder forminga sealed enclosure for a radioisotope encased therein; twothermoelectric converters each having a hot side in heat transferrelation with a respective end wall of said metal cylinder; athick-walled heat insulating cylinder, open at both ends, laterallysurrounding and containing said thin-walled meta] cylinder and saidthermoelectric converter, and substantially coaxial with saidthin-walled metal cylinder; a thick-walled metal safety capsulelaterally embracing and contacting said heat-insulating cylinder andhaving closed ends, said metal safety capsule forming a completelysealed enclosure encasing said thin-walled metal cylinder, saidthermoelectric converter and said thick-walled heatinsulated cylinder;insulated conductors for said thermoelectric converter extending insealed relation through said metal safety capsule, said thick-walledheat-insulating cylinder having end zones extending beyond saidthermoelectricconverters; respective heat bridges operatively associatedwith said end zones and forming part of said metal safety capsule; andrespective heat-collecting plates in good thermal contact with the coldsides of said thermoelectric converters; said heat bridges engaging saidheat-collecting plates to retain said thermoelectric converters inposition; said safety capsule being highly evacuated and consisting of atubular central cylinder surrounding said heatinsulating cylinder andtwo hemispherical caps welded to said central cylinder and having saidheat bridges extending inwardly therefrom; said heat bridges being inthe form of hollow cylinders defining cavities within said safetycapsule.

2. A radioisotope capsule, as claimed in claim I, in which saidheat-insulating cylinder consists of a plurality of layers of thin metalfoils of high thermal reflecting power.

3. A radioisotope capsule, as claimed in claim 1, in which saidheat-insulating cylinder consists of a fibrous heat-insulating material.

4. A radioisotope capsule, as claimed in claim 1, in which said fibrousheat-insulating material comprises quartz wool.

5. A radioisotope capsule, as claimed in claim 1, including a coat ofdepleted uranium on the exterior surface of said safety capsule, forabsorbing gamma rays. k

2. A radioisotope capsule, as claimed in claim 1, in which saidheat-insulating cylinder consists of a plurality of layers of thin metalfoils of high thermal reflecting power.
 3. A radioisotope capsule, asclaimed in claim 1, in which said heat-insulating cylinder consists of afibrous heat-insulating material.
 4. A radioisotope capsule, as claimedin claim 1, in which said fibrous heat-insulating material comprisesquartz wool.
 5. A radioisotope capsule, as claimed in claim 1, includinga coat of depleted uranium on the exterior surface of said safetycapsule, for absorbing gamma rays.